1. Field of the Invention
The present invention relates to integrated circuits and in particular to integrated active filters.
2. Description of the Prior Art
Filters for selective frequency filtering of signals are most common circuits within analog electronic systems. Along with the development of large scale integration techniques, it has become ever more important to develop techniques for efficiently implementing these filters. In many applications a large number of filters, monolithically integrated together with the ancillary circuitry for implementing certain system's functions, are required and it is desirable that the filters be wholly integrated, that they do not require adjustment and that they occupy as little area as possible on the silicon chip.
Passive filters don't lend themselves to integration for various reasons along which the inadequate precision of the R and C values in integrated form, as well as the fact that the output impedance is tied to R and C values, i.e. to the filtering function, and therefore in case a resistive and/or capacitive load is driven, this would modify the transfer function of the filter itself thus modifying both the DC gain and the cut-off frequency.
In integrated circuits, active filters employing an operational amplifier with a suitable feedback network are almost exclusively employed in order to implement the desired transfer function. Moreover in order to obviate .[.also to.]. the above recalled problem of precision of the cut-off frequency value, which remains tied to the values of the integrated R and C, the resistance R is commonly .[.substitute.]. .Iadd.substituted .Iaddend.by a capacitance Cx, switched at a frequency f.sub.s. As it is well known to the expert technician such a switched capacitor behaves electrically as a resistance having a value given by: ##EQU1##
According to the known technique it is necessary to use at least two switched capacitors and four switching for realizing a low-pass filter of the first order.